In order to promote decreases in the weight of automobiles and improve their safety, it is positively carried out to decrease the weight of steel used in automobiles to by increasing its strength. However, various types of steel sheets for use in automobiles have a press formability which decreases as their strength is increased, and this makes it difficult to carry out press forming of a steel sheet into a steel panel part having a complicated shape.
Under the circumstances, hot press forming is being increasingly utilized as a technique for obtaining high-strength steel parts having complicated shapes. In hot press forming, press working is applied to a steel sheet which is heated to a high temperature at which it is in a soft and highly ductile state, which makes it possible to form complicated shapes even with a high-strength steel sheet. In addition, by heating a steel sheet to the austenite region and then carrying out press forming of the heated steel sheet using a die followed by rapid cooling inside the die, quench hardening can be carried out at the same time as press forming, and it is possible to utilize an increase in strength due to martensitic transformation.
When hot press forming is applied to a steel sheet with no special surface coating (such as a common hot rolled or cold rolled steel sheet), if heating prior to press forming is carried out in an oxidizing atmosphere such as in air, scale (an iron oxide) is unavoidably formed on the surface of the steel sheet. Scale formed on the surface of a steel sheet easily peels off and can lead to damage of the die during press forming or to peeling of a painted film after a hot press-formed article is painted, thereby causing the corrosion resistance of the article to decrease. Therefore, in the manufacture of a hot press-formed article, it is necessary to carry out heating in a non-oxidizing atmosphere in order to suppress the formation of scale or to perform shot blasting after hot press forming in an oxidizing atmosphere in order to adequately remove scale formed on the surface of a formed article. Either method leads to an increase in manufacturing costs.
In order to suppress the formation of scale in hot press forming, it has been proposed to carry out hot press forming on a plated steel sheet having a coating on its surface. For example, Patent Documents 1-4 disclose hot press forming of a plated steel sheet having a zinc-based coating (a zinc-based plated steel sheet) which is used as a blank being formed.